1. Field of the Invention
The present invention relates to a chemical vapor deposition apparatus and in particular to a chemical vapor deposition apparatus having a supply system for bubbling a liquid raw material to vaporize it and for stably supplying the resulting raw material gas to a reaction unit.
2. Description of Prior Art
A conventional chemical vapor deposition (hereinafter referred to as CVD) apparatus having a supply system for bubbling a liquid raw material to vaporize it and supplying the resulting raw material gas to a reaction unit is shown in FIG. 1. The flow rate of a bubbling gas passing through a bubbling gas supply pipe 8 is controlled by a mass flow controller 3 and the bubbling gas is introduced into a bubbler 1. The raw material gas which has been obtained by the vaporization of the raw material due to the bubbing in the bubbler 1 is fed to a heated introduction pipe 11 (due to a heater 9) together with the bubbling gas. When film formation is carried out, a change-over valve 5 is opened and a change-over valve 6 is closed to introduce the raw material gas into a reaction unit 4 for depositing films. On the other hand, when film formation is not performed, the change-over valve 5 is closed and the change-over valve 6 is opened to directly discharge the gas through a bypass line 12 to an exhaust pipe 7 without passing through the reaction unit 4.
The prior art CVD apparatus as shown in FIG. 1 has a problem that the flow rate of the raw material gas becomes unstable when the valves 5 and 6 are switched or changed over and as a result that it becomes impossible to stably grow the film.
The instability of the raw material gas flow rate when the valves are switched is caused by a phenomenon that the pressure in the bubbler fluctuates due to the difference in conductance between a bypass line 12 and the reaction unit 4.
The flow rate Q.sub.L of the raw material gas is theoretically given by the following formula: EQU Q.sub.L =Q.sub.B .multidot.P.sub.L / (P.sub.O -P.sub.L)
wherein Q.sub.B represents the flow rate of the bubbling gas; P.sub.L represents the vapor pressure of the liquid raw material; and P.sub.O represents the pressure in the bubber. The flow rate Q.sub.B of the bubbling gas is controlled by the mass flow controller 3 and the vapor pressure P.sub.L of the liquid raw material is controlled by controlling the temperature of the liquid raw material. In the prior art, however, a change in the conductance at the side of introduction pipe 11 causes the fluctuation of the pressure P.sub.O in the bubbler 1, resulting in an instability of the raw material gas flow rate Q.sub.L. In order to obtain the stable flow rate of the raw material gas, it is necessary to suppress the fluctuations of the pressure in the bubbler 1 caused by the change in pressure in the reaction unit 4.